Using fast switching power semiconductor devices, such as insulated gate bipolar transistors (IGBTs), a pulse width modulation (PWM) inverter can be operated at high frequency (for example, up to 20 kHz), significantly improving motor performance. High-performance induction motor drives require such fast switching transition and high switching frequency from PWM inverters to fully realize such advantages as fast dynamic response, high efficiency, and low acoustic noise.
There are some perceived problems with high-speed switching, however. During the high-speed switching of the IGBTs, charging and discharging current can sometimes flow through a complex distributed capacitance inherently associated with the switching system. Such transient charging/discharging current can flow through the distributed capacitance to the motor and return to the power mains along various paths. This well-known and well-understood phenomenon is typically referred to as "high dv/dt common mode current" or "high dv/dt common mode voltage," and is generally regarded as undesirable.
High dv/dt common mode voltages and currents caused by high-speed switching can lead to premature motor winding insulation failure and/or motor bearing failure. The high speed voltage and/or current changes which lead to high dv/dt common mode voltages and currents occur at every switching instant of the IGBTs; hence, the phenomenon worsens as switching frequency increases.
Experimental investigation has shown that common-mode dv/dt generated by PWM voltage source inverting (VSI) is closely related to shaft voltage and bearing current in a driven motor. (As would be appreciated by those of ordinary skill in the art, "bearing current" refers to spurious and undesirable current flow through motor bearings when operated under pure sine wave or PWM inverter sources. See, e.g., Erdman, et al., "Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages," IEEE Trans. Industry Applications, vol. 32, no. 2, pp. 250-259, 1995. Premature bearing failure is related to the bearing current resulting from the interaction between common-mode voltage and parasitic capacitance in the motor.
Passive solutions have been sought for the problems associated with differential mode and common mode dv/dt in PWM induction motor drive systems. See, e.g., Rendusara et al., "New Inverter Output Filter Configuration Reduces Common Mode and Differential Mode dv/dt at the Motor Terminals in PWM Drive Systems," Record of the 28th IEEE Power Electronics Specialists Conference (PESC '97), Jun. 22-27, 1997, pp. 1269-1275.
There have been various proposed other passive methods of dealing with the problems of common mode and differential mode dv/dt, including shaft grounding, bearing isolation, and conductive grease. None of these techniques has proven to be entirely effective.
The above-cited Erdman et al. reference proposes shielding the stator winding as a method of address problems with bearing currents. This method, while possibly effective, would tend to be undesirably expensive in practice.
In addition to passive solutions, active solutions have also been proposed. Sinha et al., "A Four Level Rectifier-Inverter System for Drive Applications," Conf. Record of IAS 96, pp. 980-981 (1996) proposes using an additional phase leg to control the neutral point voltage of induction motors. See also, Lipo et al., "Elimination of Common Mode Voltage in Three Phase Sinusoidal Power Converters," Proceedings of PESC '96--26th Annual IEEE Power Electronics Specialists Conference, Baveno, Italy, Jun. 24-27, 1996, pp. 1968-1972.
Seeking to reduce the common-mode ground leakage current of PWM inverter-fed induction motors, there has been proposed a series of methods based on the common mode choke and transformer. See, e.g., Ogasawara et al., "Modeling and Damping of High-Frequency Leakage Currents in PWM Inverter-Fed AC Motor Drive Systems," IEEE Trans. on Industry Applications, vol. 32, no. 5, 1996, pp. 1105-1114, and Ogasawara et al., "An Active Circuit for Cancellation of Common-Mode Voltage Generated by a PWM Inverter," Record of the 28th Annual IEEE Power Electronics Specialists Conference, vol. 2, 1997, pp. 1547-1553. The "active common noise canceller" proposed in this latter Ogaswara et al. reference appears to show the successful cancellation of common mode voltages. However, this approach employs an emitter follower, which can be difficult to implement for high-voltage applications.